Propagation and Detection of Light Beams Carrying Orbital Angular Momentum
Kaitlyn Morgan, Clemson University
This week’s OSA Incubator, Propagation and Detection of Light Beams Carrying Orbital Angular Momentum, has been an incredible experience so far. It is wonderful seeing so many people in the same room, eager to discuss the science and applications of orbital angular momentum (OAM). OAM is a unique measurable phenomenon that light possesses when the wavefront has a helical twist. This relatively new phenomenon has rapidly been gaining interest since its discovery in 1992 by Les Allen, and many people are discovering unique applications of these spatial modes. This Incubator is all about discussing the future direction of OAM applications and research.
The hosts of this meeting are Brandon Cochenour and Linda Mullen from the Naval Air Warfare Center, Eric Johnson from Clemson University, and Peter Morrison from the Office of Naval Research. They kicked off the meeting by saying how they hoped to inspire collaborations and initiate an in-depth discussion of the future applications and issues using OAM light modes, especially for free-space links in air and underwater. Mike Wardlaw gave a wonderful introduction covering the current state of marine optical links and the issues with these systems. His talk clarified the interest in bringing OAM to these environments, particularly the hope that we can push limitations in bandwidth and transmission distance compared to that of traditional optical links.
The first topic session of the day covered Propagation and Interaction with Media, focusing primarily on turbulence. Martin Lavery, University of Glasgow, started the session by discussing how the effects of propagation through turbulence affect the recoverability and crosstalk of OAM optical modes. He showed distortion and mode mixing of the optical signal through kilometers of air. He also showed minimal phase distortion through fast flowing turbid water. Greg Gbur, UNC Charlotte, discussed the difference between OAM measurement versus the topological charge as a beam propagates through turbulence, and focused on the phase profile of such modes. He also introduced fractional OAM modes and partially coherent modes as potential alternative solutions to transmitting traditional OAM signals though these environments. Darryl Sanchez gave us a great history of OAM and showcased measurements of OAM generated in distributed volume turbulence through various environments including vertical astronomical measurements and horizontal measurements through a desert environment. Shermineh Farchild (pictured here), University of Central Florida, CREOL, introduced high-powered lasers and filamentation into the propagation discussion, and showed the group’s facility capabilities including an aerosol chamber and an outdoor propagation range. Filamentation is a non-linear process where the beam self-focuses through the environment, creating a stable beam of extremely high power density which generates a plasma in the medium.
The second session, Devices and Fabrication, covered a variety of methods used to generate and recover OAM modes and the applications of those methods. Host Eric Johnson (pictured here) demonstrated the use of various wafer-based optics to generate and test properties of OAM modes. His talk highlighted methods of dynamically modulating the spatial beam profiles using a specific architecture for generating ring modes. Juliet Gopinath’s talk focused on generating and tuning modes in polarization-maintaining fiber and taught us how to do the twist by measuring OAM states using cylindrical lenses. When discussing her work at the University of Colorado, Bolder, she also demonstrated imaging with extremely high resolution, which she called ‘super imaging’. Rodrigo Correa, University of Central Florida, CREOL, talked about using spatially diverse fibers in photonic lanterns as a spatial multiplexing device, and how to excite specific modes in the device. These devices can even be used to generate arrays of structured modes. Siddharth Ramachandran, Boston University, discussed the using an annular core as the ideal waveguide for the propagation of OAM modes and gave a delightful metaphor for OAM, relating the optical rotation to the rotation of a bicycle wheel or spinning top. He also discussed the geometric phase which arises from propagation along a 3D path.
Today we discussed the generation, propagation, detection, and potential applications for using OAM modes in free space (air) and underwater. So far it has sparked several interesting discussions, particularly about practical limitations when using these mode sets. I look forward to what the next two sessions will bring. Tomorrow’s blog will cover the topics of Communications and Sensing and Imaging, as well as what is next for this community, so stay tuned for the next post.
The Incubator Participants
The Incubator included lively discussions throughout the program.